A wide-bandgap semiconductor has a higher breakdown voltage than a silicon semiconductor and can make a huge amount of current flow through it, and therefore, has attracted a lot of attention as a semiconductor material that could be potentially used to make a power device. Among other things, a silicon carbide semiconductor that uses silicon carbide (SiC) has a much higher dielectric breakdown voltage than any other wide-bandgap semiconductor, and therefore, is expected to be the best semiconductor material to make a next-generation low-loss power device. Examples of semiconductor devices that use a silicon carbide semiconductor include a metal-insulator-semiconductor field-effect transistor (MISFET) that is a unipolar switching device. A metal-oxide-semiconductor field-effect transistor (MOSFET) is one of those MISFETs. An SiC-MOSFET has attracted a lot of attention these days as a key semiconductor device which can operate at high speeds and which will contribute to reducing the size and loss of a power unit.
An SiC-MOSFET has a channel mobility which is significantly lower than the theoretical limit, which is problem with the SiC-MOSFET. They believe that an SiC-MOSFET has that low channel mobility because there should be interface states and a lot of other defects in the interface between the silicon carbide semiconductor and a silicon dioxide (SiO2) film. Thus, to reduce the number of those defects in the interface between the oxide film and the silicon carbide semiconductor, someone proposed that a nitridation process be performed after an oxide film has been formed by either thermal oxidation or chemical vapor deposition (CVD) process. By heavily doping the interface between an oxide film and a silicon carbide semiconductor with nitrogen, the density of those interface states should be reduced and the channel mobility should be increased. For example, Non-Patent Document No. 1 discloses that a heat treatment is carried out at 1175° C. for two hours within a nitrogen gas to introduce nitrogen into the interface between the silicon oxide film and the silicon carbide semiconductor at an area concentration of 2×1014 cm−2 or more.
Meanwhile, Patent Document No. 1, for example, discloses that when a negative bias is applied to the gate electrode of an SiC-MOSFET in OFF state, its threshold voltage (Vth) will vary with time (more specifically, will shift in the negative direction). Patent Document No. 1 says that such a phenomenon could be produced, because holes (which are positive electric charges) would gradually get caught in hole traps that should be present at a high density in the vicinity of the interface between the silicon oxide film and the silicon carbide semiconductor. Patent Document No. 1 also proposes that in order to suppress such a variation, the concentration of nitrogen atoms in the interface between the silicon carbide semiconductor and the silicon oxide film be set to be lower (at less than 1.6×101′ cm−2) than in Non-Patent Document No. 1.
It should be noted that it is known that even in a p-channel MOSFET which uses a silicon (Si) semiconductor, when a negative bias is applied to its gate electrode, its threshold voltage will also vary (which is called an NBTI (negative bias temperature instability)). The variation in threshold voltage will be 0.1 V or less under a stress voltage applied for a long time (e.g., 1000 h). However, they say that this phenomenon would be produced by the movement of carriers into and out of interface states due to the presence of dangling bonds of Si (see Patent Document No. 2, for example), which is quite different from the cause of the phenomenon to be observed in the SiC-MOSFET.